release: Update few release dates

Some of the dates on which releases were done or are planned
didn't match with exact release dates. So, correct them to
show the actual release dates.

Signed-off-by: Ruchika Gupta <ruchika.gupta@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>

general/releases.rst

@@ -28,11 +28,11 @@ old, upcoming or future release.
 +---------------+--------------+
 | OP-TEE 3.16.0 | 14/Jan/22    |
 +---------------+--------------+
-| OP-TEE 3.15.0 | 15/Oct/21    |
+| OP-TEE 3.15.0 | 18/Oct/21    |
 +---------------+--------------+
 | OP-TEE 3.14.0 | 16/July/21   |
 +---------------+--------------+
-| OP-TEE 3.13.0 | 16/Apr/21    |
+| OP-TEE 3.13.0 | 30/Apr/21    |
 +---------------+--------------+
 | OP-TEE 3.12.0 | 20/Jan/21    |
 +---------------+--------------+