Fix typos

Closes GH-316.

Author: tbroadley

doc/plugins.md

@@ -120,7 +120,7 @@ Have a good idea for a new plugin?  Let’s [chat][gitter] and make it happen!
     — Convert links to local or remote OpenAPI definition to tables with
     summaries of all paths
 *   [`remark-ping`](https://github.com/zestedesavoir/zmarkdown/tree/master/packages/remark-ping)
-    — Custom syntax, parses `@user`, `@**first last**`, configurable existance
+    — Custom syntax, parses `@user`, `@**first last**`, configurable existence
     check.  Rehype compatible
 *   [`remark-react`](https://github.com/mapbox/remark-react)
     — Compile Markdown to [React](https://github.com/facebook/react)

packages/remark-parse/readme.md

@@ -390,7 +390,7 @@ The given `node`.
 ### Turning off a tokenizer
 
 In rare situations, you may want to turn off a tokenizer to avoid parsing
-that syntactic feature.  This can be done by deleting the tokenzier from
+that syntactic feature.  This can be done by deleting the tokenizer from
 your Parser’s `blockTokenizers` (or `blockMethods`) or `inlineTokenizers`
 (or `inlineMethods`).