docs: Restore literal tabs to the TSV examples in the rST source

These were inadvertently lost in "Convert markdown pages to
reStructuredText" (7b8e45e) which converted them to spaces.

Note that these tabs still get expanded to spaces when rendering the rST
to HTML, and thus copying the example from the HTML page to paste into a
file will not produce a TSV.  I'll address this in a subsequent commit.

docs/advanced-functionality/drag-drop-csv-tsv.rst

@@ -14,11 +14,11 @@ A TSV file as follows can be dragged onto `nextstrain.org/zika <https://nextstra
 
 .. code:: text
 
-   strain  secret
-   USVI/19/2016    A
-   USVI/28/2016    B
-   USVI/41/2016    C
-   USVI/42/2016    C
+   strain	secret
+   USVI/19/2016	A
+   USVI/28/2016	B
+   USVI/41/2016	C
+   USVI/42/2016	C
 
 .. figure:: ../assets/csv-extra-data.png
    :alt: auspice with extra data shown via csv
@@ -29,11 +29,11 @@ A more complex metadata file may look like the following, which makes use of add
 
 .. code:: text
 
-   strain  secret  secret__colour  latitude    longitude
-   USVI/19/2016    A   #f4e409 0   -120
-   USVI/28/2016    B   #f49015 0   -115
-   USVI/41/2016    C   #710000 0   -100
-   USVI/42/2016    C   #710000 0   -120
+   strain	secret	secret__colour	latitude	longitude
+   USVI/19/2016	A	#f4e409	0	-120
+   USVI/28/2016	B	#f49015	0	-115
+   USVI/41/2016	C	#710000	0	-100
+   USVI/42/2016	C	#710000	0	-120
 
 .. figure:: ../assets/csv-extra-data-2.png
    :alt: auspice with extra data shown via csv