fix missing mod tag

Author: lemontron

lemontron-rev-a/bom/hardware/MGN9C.md

@@ -8,5 +8,7 @@ modification: true
 The smallest rail in the BOM allows the tool head to travel left and right.
 
 ## Required Modification
+
 Trim off one hole. Suggested to use a [Dremel](https://www.amazon.com/dp/B0D2D28FSV?tag=lemontron-20) & cutoff wheel or
-a hacksaw. The rail will wind up around 230mm.
+a hacksaw. The rail will wind up around 230mm. Don't cut too close to the holes! You'll expose too much of the rail and
+it will scrape the belt!

lemontron-rev-e/bom/hardware/MGN9C.md

@@ -8,5 +8,7 @@ modification: true
 The smallest rail in the BOM allows the tool head to travel left and right.
 
 ## Required Modification
+
 Trim off one hole. Suggested to use a [Dremel](https://www.amazon.com/dp/B0D2D28FSV?tag=lemontron-20) & cutoff wheel or
-a hacksaw. The rail will wind up around 230mm.
+a hacksaw. The rail will wind up around 230mm. Don't cut too close to the holes! You'll expose too much of the rail and
+it will scrape the belt!