Update doc/specs/stdlib_stats_distribution_exponential.md

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

Author: Jim-215-Fisher

doc/specs/stdlib_stats_distribution_exponential.md

@@ -163,7 +163,7 @@ program demo_exponential_pdf
 end program demo_exponential_pdf
 ```
 
-## `cdf_exp` - exponential distribution cumulative distribution function
+## `cdf_exp` - exponential cumulative distribution function
 
 ### Status