Fixing small `an` to `a` typo in Maybe.elm
Fixing small `an` to `a` typo in Maybe.elm

```markdown
Transform an `Maybe` value with a given function:
```

looks like it was intended to be

```markdown
Transform a `Maybe` value with a given function:
```

in the comments so just giving it a quick adjustment.
simonh1000/core,laszlopandy/core,laszlopandy/core,colinmccabe/core,Pisys/core,alexiamcdonald/elm-core,rgrempel/core,martinos/core,rick68/core,alexiamcdonald/elm-core,suryagaddipati/core,Dandandan/core,simonh1000/core,rick68/core,suryagaddipati/core,martinos/core,Dandandan/core,colinmccabe/core,bradurani/core,eeue56/core,ThomasWeiser/core,rgrempel/core,bradurani/core,ThomasWeiser/core,Pisys/core,eeue56/core