ChangeLog.md: Add colons to sub-headers

For some reason, I failed to add a colon to the "Significant changes
relative to 2.1 beta1" sub-header, and the mistake propagated from
there.

ChangeLog.md

@@ -1,7 +1,7 @@
 2.1.4
 =====
 
-### Significant changes relative to 2.1.3
+### Significant changes relative to 2.1.3:
 
 1. Fixed a regression introduced in 2.1.3 that caused build failures with
 Visual Studio 2010.
@@ -36,7 +36,7 @@ virtual array access") under certain circumstances.
 2.1.3
 =====
 
-### Significant changes relative to 2.1.2
+### Significant changes relative to 2.1.2:
 
 1. Fixed a regression introduced by 2.0 beta1[7] whereby cjpeg compressed PGM
 input files into full-color JPEG images unless the `-grayscale` option was
@@ -60,7 +60,7 @@ be reproduced using the libjpeg API, not using djpeg.
 2.1.2
 =====
 
-### Significant changes relative to 2.1.1
+### Significant changes relative to 2.1.1:
 
 1. Fixed a regression introduced by 2.1 beta1[13] that caused the remaining
 GAS implementations of AArch64 (Arm 64-bit) Neon SIMD functions (which are used
@@ -92,7 +92,7 @@ image contains incomplete or corrupt image data.
 2.1.1
 =====
 
-### Significant changes relative to 2.1.0
+### Significant changes relative to 2.1.0:
 
 1. Fixed a regression introduced in 2.1.0 that caused build failures with
 non-GCC-compatible compilers for Un*x/Arm platforms.
@@ -121,7 +121,7 @@ transform a specially-crafted malformed JPEG image.
 2.1.0
 =====
 
-### Significant changes relative to 2.1 beta1
+### Significant changes relative to 2.1 beta1:
 
 1. Fixed a regression introduced by 2.1 beta1[6(b)] whereby attempting to
 decompress certain progressive JPEG images with one or more component planes of