Use proper file extension for uniq command docs (#1411)

docs/commands/uniq.md

@@ -0,0 +1,36 @@
+# uniq
+
+Returns unique rows or values from a dataset.
+
+## Examples
+
+Given a file `test.csv`
+
+```
+first_name,last_name,rusty_at,type
+Andrés,Robalino,10/11/2013,A
+Andrés,Robalino,10/11/2013,A
+Jonathan,Turner,10/12/2013,B
+Yehuda,Katz,10/11/2013,A
+```
+
+```
+> `open test.csv | uniq`
+━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━
+ # │ first_name │ last_name │ rusty_at   │ type
+───┼────────────┼───────────┼────────────┼──────
+ 0 │ Andrés     │ Robalino  │ 10/11/2013 │ A
+ 1 │ Jonathan   │ Turner    │ 10/12/2013 │ B
+ 2 │ Yehuda     │ Katz      │ 10/11/2013 │ A
+━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━
+```
+
+```
+> `open test.csv | get type | uniq`
+━━━┯━━━━━━━━━
+ # │ <value>
+───┼─────────
+ 0 │ A
+ 1 │ B
+━━━┷━━━━━━━━━
+```