Update doc/specs/stdlib_math.md

Co-authored-by: Federico Perini <federico.perini@gmail.com>

Author: MilanSkocic

doc/specs/stdlib_math.md

@@ -404,7 +404,7 @@ Elemenal function.
 
 #### Return value
 
-Returns the `real` phase in radian
+Returns the `real` phase angle in radians
 
 #### Example