Fix typo in documentation (#33064)

Author: kdheepak

doc/src/manual/metaprogramming.md

@@ -1295,7 +1295,7 @@ to build some more advanced (and valid) functionality...
 
 ### An advanced example
 
-Julia's base library has a an internal `sub2ind` function to calculate a linear index into an n-dimensional
+Julia's base library has an internal `sub2ind` function to calculate a linear index into an n-dimensional
 array, based on a set of n multilinear indices - in other words, to calculate the index `i` that
 can be used to index into an array `A` using `A[i]`, instead of `A[x,y,z,...]`. One possible implementation
 is the following: