HACKING: fix minor typos

s/additonal/additional/
s/seperately/separately/
Use uppercase after a '.'

Change-Id: I933894ae44924363d634fa07d3de4ceeb4b329e2
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5162
Tested-by: jenkins
Reviewed-by: Jonathan McDowell <noodles-openocd@earth.li>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

HACKING

@@ -77,7 +77,7 @@ patch:
   src/openocd -s ../tcl -f /path/to/openocd.cfg
   @endcode
 
-Please consider performing these additonal checks where appropriate
+Please consider performing these additional checks where appropriate
 (especially Clang Static Analyzer for big portions of new code) and
 mention the results (e.g. "Valgrind-clean, no new Clang analyzer
 warnings") in the commit message.
@@ -148,7 +148,7 @@ wget http://openocd.zylin.com/tools/hooks/commit-msg
 mv commit-msg .git/hooks
 chmod +x .git/hooks/commit-msg
 @endcode
-  \note A script exists to simplify the two items above. execute:
+  \note A script exists to simplify the two items above. Execute:
 @code
 tools/initial.sh <username>
 @endcode
@@ -160,7 +160,7 @@ git config --global user.email "john@smith.org"
 @endcode
 -# Work on your patches. Split the work into
    multiple small patches that can be reviewed and
-   applied seperately and safely to the OpenOCD
+   applied separately and safely to the OpenOCD
    repository.
 @code
 while(!done) {