Update CONTRIBUTING.md

Fixed spelling

CONTRIBUTING.md

 * Please make an individual pull request for each suggestion.
 * Use the following format for libraries: \[LIBRARY\]\(LINK\) - DESCRIPTION.
 * Prefix duplicate library names with their vendor or namespace followed by a space: Foo\Bar would be Foo Bar.
-* New categories, or improvements to the existing categorisaton are welcome.
+* New categories, or improvements to the existing categorisation are welcome.
 * Keep descriptions short and simple. 
 * End all descriptions with a full stop/period.
 * Check your spelling and grammar.
 * Make sure your text editor is set to remove trailing whitespace.
 
-Thank you for your suggestions!
+Thank you for your suggestions!