docs: Match Single Blank Line Above and Below A Title

Signed-off-by: Yifei Ding <yifeiding@protonmail.com>

docs/skew_correction.md

@@ -7,6 +7,7 @@ first use mechanical means to get your printer as square as possible prior
 to applying software based correction.
 
 ## Print a Calibration Object
+
 The first step in correcting skew is to print a
 [calibration object](https://www.thingiverse.com/thing:2563185/files)
 along the plane you want to correct.  There is also a
@@ -19,6 +20,7 @@ do this by either removing the [skew_correction] module from printer.cfg
 or by issuing a `SET_SKEW CLEAR=1` gcode.
 
 ## Take your measurements
+
 The [skew_correcton] module requires 3 measurements for each plane you want
 to correct; the length from Corner A to Corner C, the length from Corner B
 to Corner D, and the length from corner A to corner D.  When measuring length
@@ -28,6 +30,7 @@ AD do not include the flats on the corners that some test objects provide.
 
 ## Configure your skew
 Make sure [skew_correction] is in printer.cfg.  You may now use the `SET_SKEW`
+
 gcode to configure skew_correcton.  For example, if your measured lengths
 along XY are as follows:
 ```
@@ -72,6 +75,7 @@ After removing a profile you will be prompted to issue a `SAVE_CONFIG` to
 make this change persist.
 
 ## Verifying your correction
+
 After skew_correction has been configured you may reprint the calibration
 part with correction enabled.  Use the following gcode to check your
 skew on each plane.  The results should be lower than those reported via