Update src/stdlib_stats_distribution_exponential.fypp

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

Author: Jim-215-Fisher

src/stdlib_stats_distribution_exponential.fypp

@@ -41,7 +41,7 @@ module stdlib_stats_distribution_exponential
 
 
     interface pdf_expon
-    !! Version experimental
+    !! Version: experimental
     !!
     !! Exponential Distribution Probability Density Function
     !! ([Specification](../page/specs/stdlib_stats_distribution_exponential.html#