Update Expressions.rmd

Expressions.rmd

@@ -650,7 +650,7 @@ The real `source()` is considerably more complicated because it can `echo` input
 
 ## Walking the AST with recursive functions {#ast-funs}
 
-It's easy to modify sa single call with `substitute()` or `pryr::modify_call()`. For more complicated tasks we need to work directly with the AST. The base `codetools` package provides some useful motivating examples of how we can do this: \index{recursion!over ASTs}
+It's easy to modify a single call with `substitute()` or `pryr::modify_call()`. For more complicated tasks we need to work directly with the AST. The base `codetools` package provides some useful motivating examples of how we can do this: \index{recursion!over ASTs}
 
 * `findGlobals()` locates all global variables used by a function. This
   can be useful if you want to check that your function doesn't inadvertently