docs: fix some typos in file-specifiers spec (#15)

PR-URL: #15
Credit: @Mstrodl
Reviewed-By: @zkat

Author: Mstrodl

doc/spec/file-specifiers.md

@@ -14,7 +14,7 @@ URLs and URL-like strings for other types.
   slashes on a file specifier will be removed, that is 'file://../foo/bar`
   references the same package as same as `file:../foo/bar`.  The latter is
   considered canonical.
-* Attempting to install a specifer that has a windows drive letter will
+* Attempting to install a specifier that has a windows drive letter will
   produce an error on non-Windows systems.
 * A valid `file:` specifier points is:
   * a valid package file. That is, a `.tar`, `.tar.gz` or `.tgz` containing
@@ -64,7 +64,7 @@ down the destination package's `node_modules` you should create a shrinkwrap for
 separately.
 
 This is necessary to support the mono repo use case where many projects file
-to the same package.  If each project included its own npm-shrinkwrap.json
+to the same package.  If each project included its own `npm-shrinkwrap.json`
 then they would each have their own distinct set of transitive dependencies
 and they'd step on each other any time you ran an install in one or the other.
 
@@ -75,7 +75,7 @@ shrinkwrapped packages.
 
 #### File type specifiers pointing at tarballs
 
-File-type specifiers pointing at a `.tgz` or `.tar.gz or `.tar` file will
+File-type specifiers pointing at a `.tgz` or `.tar.gz` or `.tar` file will
 install it as a package file in the same way we would a remote tarball.  The
 checksum of the package file should be recorded so that we can check for updates.