Fix stdout and stderr occurences

Fix stdout and stderr occurences in documentation and contribution
guidelines.

CONTRIBUTING.md

@@ -51,7 +51,7 @@ Here are the things you should do when creating an issue:
    let g:ycm_server_log_level = 'debug'
    ```
 
-   Run `:YcmToggleLogs Stderr` in vim to open the logfile. Attach the contents
+   Run `:YcmToggleLogs stderr` in vim to open the logfile. Attach the contents
    of this file to your issue.
 3. Add the output of the `:YcmDebugInfo` command.
 4. **Create a test case for your issue**. This is critical. Don't talk about how

README.md

@@ -875,7 +875,7 @@ completion engine.
 
 This command automatically opens in windows the stdout and stderr logfiles
 written by the `ycmd` server. If one or both logfiles are already opened, they
-are automatically closed. `Stderr` or `Stdout` can be specified as an argument
+are automatically closed. `stderr` or `stdout` can be specified as an argument
 of this command to only open the corresponding logfile instead of both. If this
 logfile is already opened, it will be closed. Only for debugging purpose.
 

doc/youcompleteme.txt

@@ -1087,7 +1087,7 @@ The *:YcmToggleLogs* command
 
 This command automatically opens in windows the stdout and stderr logfiles
 written by the 'ycmd' server. If one or both logfiles are already opened, they
-are automatically closed. 'Stderr' or 'Stdout' can be specified as an argument
+are automatically closed. 'stderr' or 'stdout' can be specified as an argument
 of this command to only open the corresponding logfile instead of both. If this
 logfile is already opened, it will be closed. Only for debugging purpose.