Update stdlib_math.md

Co-authored-by: Jeremie Vandenplas <jeremie.vandenplas@gmail.com>

Author: MilanSkocic

doc/specs/stdlib_math.md

@@ -406,7 +406,7 @@ Elemenal function.
 
 #### Return value
 
-Returns the `real` phase angle in radians
+Returns the `real` phase angle in radians.
 
 #### Example