H2: Fix titles

exercises/h2.md

@@ -70,7 +70,7 @@ phelenius@my-machine:~$ cat /var/log/apache2/access.log
 
 Error logs of an Apache server can be found (by default) at `/var/log/apache2/error.log`. HTTP status codes with brief description are defined on [Apache homepage - Common HTTP Status Codes](https://wiki.apache.org/httpd/CommonHTTPStatusCodes).
 
-#### Other error situations - libraries & typos
+#### Other error situations - libraries, headers & typos
 
 The most common errors I have encountered on Linux desktop usage as a system administrator are various [Permission denied events](https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Step_by_Step_Guide/s1-navigating-ownership.html) mostly as a consequence of a human error (forgetting sudo or something similar) or various typos in commands. I have compiled a lot of programs from source code, making various library errors practically unavoidable in long-term. The syntax of these errors is: [Cannot open shared object file: No such file or directory](https://stackoverflow.com/questions/480764/linux-error-while-loading-shared-libraries-cannot-open-shared-object-file-no-s/480786) where an executable can't find a requested `.so` library file or version from library path. Both of these errors can be avoided by typing commands precisely and avoiding compiling of software (and relying on official repositories and system-wide package upgrades, considering that installed packages are maintained actively).
 
@@ -80,7 +80,7 @@ When compiling a program, common problems include
 
 - missing header files (`.h` suffix), located at the same folder with original `.c` or `.cpp` files or at `/usr/include` folder or subfolders.
 
-#### Other error situations - unsupported Linux OS versions & breaking libraries
+#### Other error situations - unsupported Linux OS versions & broken libraries
 
 In long-term, using Linux distributions such as Ubuntu (or any variant that relies on Ubuntu package repositories, such as Linux Mint) become unusable with newer software. The newer versions require more recent library versions which older Ubuntu releases can't possibly provide anymore because official repositories have been shutted down. In this state, either older software must be used (not recommended) on older operating system or the system must be upgraded to more recent version where the newer packages are still available. In some extent, compiling programs from source on an older operating system version is possible and thus life cycle of an old operating system installation can be extended for some programs. This approach, however, leads easily to security risks and bugs which have not been fixed, and especially the security part is essential on Linux server installations.