doc: fix comma splice in Assertion Testing doc

This fixes a minor typographical error in the Assertion Testing doc.

PR-URL: https://github.com/nodejs/node/pull/2728
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>

doc/api/assert.markdown

@@ -11,7 +11,7 @@ Throws an exception that displays the values for `actual` and `expected` separat
 
 ## assert(value[, message]), assert.ok(value[, message])
 
-Tests if value is truthy, it is equivalent to `assert.equal(true, !!value, message);`
+Tests if value is truthy. It is equivalent to `assert.equal(true, !!value, message)`.
 
 ## assert.equal(actual, expected[, message])