doc: update cross compiler machine for Linux armv7

Update cross compiler machine for Linux armv7 to Ubuntu 18.04.

Signed-off-by: Richard Lau <riclau@uk.ibm.com>

PR-URL: #32812
Refs: nodejs/build#2168
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

BUILDING.md

@@ -171,7 +171,7 @@ Binaries at <https://nodejs.org/download/release/> are produced on:
 | aix-ppc64             | AIX 7.1 TL05 on PPC64BE with GCC 6                                       |
 | darwin-x64 (and .pkg) | macOS 10.15, Xcode Command Line Tools 11 with -mmacosx-version-min=10.13 |
 | linux-arm64           | CentOS 7 with devtoolset-8 / GCC 8 <sup>[8](#fn8)</sup>                  |
-| linux-armv7l          | Cross-compiled on Ubuntu 16.04 x64 with [custom GCC toolchain](https://github.com/rvagg/rpi-newer-crosstools)   |
+| linux-armv7l          | Cross-compiled on Ubuntu 18.04 x64 with [custom GCC toolchain](https://github.com/rvagg/rpi-newer-crosstools)   |
 | linux-ppc64le         | CentOS 7 with devtoolset-8 / GCC 8 <sup>[8](#fn8)</sup>                  |
 | linux-s390x           | RHEL 7 with devtoolset-8 / GCC 8 <sup>[8](#fn8)</sup>                    |
 | linux-x64             | CentOS 7 with devtoolset-8 / GCC 8 <sup>[8](#fn8)</sup>                  |