Correct typeof reference in README

JavaScript's typeof operator returns "string" not "String" for string values and objects

README.md

@@ -240,7 +240,7 @@ Type: `Boolean | String | undefined`
 Default: `undefined`
 **Special:** Setting the `sourceMap` option requires also setting the `outFile` option
 
-Enables the outputting of a source map during `render` and `renderSync`. When `sourceMap === true`, the value of `outFile` is used as the target output location for the source map. When `typeof sourceMap === "String"`, the value of `sourceMap` will be used as the writing location for the file.
+Enables the outputting of a source map during `render` and `renderSync`. When `sourceMap === true`, the value of `outFile` is used as the target output location for the source map. When `typeof sourceMap === "string"`, the value of `sourceMap` will be used as the writing location for the file.
 
 ### sourceMapContents
 Type: `Boolean`