DMA-API: Spelling s/This/Think/

Also remove spaces before TABs while we're at it.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DMA-API-HOWTO.txt

@@ -353,7 +353,7 @@ There are two types of DMA mappings:
   transfer, unmapped right after it (unless you use dma_sync_* below)
   and for which hardware can optimize for sequential accesses.
 
-  This of "streaming" as "asynchronous" or "outside the coherency
+  Think of "streaming" as "asynchronous" or "outside the coherency
   domain".
 
   Good examples of what to use streaming mappings for are: