Fix documentation errors (preservim#1269)

* Fix a typo: "NERDTree_*" -> "NERD_Tree_*"

* Format text

* Use the proper abbreviation for the BEL character

* Format text

* Make the documentation match the code

The syntax file sets 'conceallevel' to 2, not 3.

* Update the "CHANGELOG.md" file

Author: lifecrisis

CHANGELOG.md

@@ -5,6 +5,7 @@
         - **.PATCH**: Pull Request Title (PR Author) [PR Number](Link to PR)
 -->
 #### 6.10
+- **.16**: Fix documentation errors. (lifecrisis) [#1269](https://github.com/preservim/nerdtree/pull/1269)
 - **.15**: Ensure backward compatible testing of types. (lifecrisis) [#1266](https://github.com/preservim/nerdtree/pull/1266)
 - **.14**: Replace trim() with a version-compatible alternative. (PhilRunninger) [#1265](https://github.com/preservim/nerdtree/pull/1265)
 - **.13**: Change highlighting of bookmarks in the tree. (PhilRunninger) [#1261](https://github.com/preservim/nerdtree/pull/1261)

doc/NERDTree.txt

@@ -1256,10 +1256,10 @@ responsible pull request: https://github.com/preservim/nerdtree/pull/868.
 The default value of this variable depends on the features compiled into your
 vim and the values of |NERDTreeDirArrowCollapsible| and
 |NERDTreeDirArrowExpandable|.
-  * If your vim is compiled with the +conceal feature, it is the "\x07" (BELL)
-    character, and it is hidden by setting 'conceallevel' to 3. If you use
-    autocommands, make sure none of them change that setting in the NERDTree_*
-    buffers.
+  * If your vim is compiled with the +conceal feature, it is the "\x07"
+    (BEL) character, and it is hidden by setting 'conceallevel' to 2. If you
+    use autocommands, make sure none of them change that setting in the
+    NERD_Tree_* buffers.
   * If your vim does NOT have the +conceal feature and you're using "\u00a0"
     (non-breaking space) to hide the directory arrows, "\u00b7" (middle dot)
     is used as the default delimiter.